If we have become a socialist nation by then, he'll be remembered as someone who was instrumental in beginning to claw us away from capitalism.
If we are a still a capitalist nation, he'll be remembered as a devoted activist for crazy ideas that we wisely never enacted into law.
The fight for Bernie's ideas will probably live on even after his death. This isn't Bernie Sanders' alone - Universal healthcare (public one) has been a fight of the Dems for almost a 100 years - From FDR to Harry Truman to Ted Kennedy. Tuition fees used to be free or very cheap. Taxes for almost 50 years used to much MUCH higher.
It is highly unlikely when every major nation has universal healthcare that the fight for it will die down after Bernie & those ideas will be looked as crazy !